The legacy of Mark Rothko, one of the 20th century's leading artists, is coming home to Portland.

The Portland Art Museum on Thursday announced that as part of a major expansion, it will construct a three-story, glass-walled Rothko Pavilion that will connect the museum's two buildings in the South Park Blocks. The pavilion will house major Rothko paintings that will be loaned by the artist's children, Christopher Rothko and Kate Rothko Prizel, on a rotating basis during a 20-year partnership.

The Rothko Pavilion is the cornerstone of a $75 million capital and endowment campaign that the museum also announced Thursday. In a prepared statement, the museum said it has raised $21.75 million toward its $50 million capital goal and $5.4 million toward its $25 million endowment goal.

Rothko was born Marcus Rothkowitz in 1903 in Latvia, in eastern Europe. His family immigrated to the United States in 1913, settling in Portland. He attended Lincoln High School and as a teenager took classes at the Portland Art Museum, where he had his first solo museum exhibition in 1933.

"The partnership with the Rothko family is a homecoming of sorts, enabling us to share with the public major works from the family's private collection, offer new insight into Rothko's practice, and honor his legacy in the Pacific Northwest and the international arts community," said Brian Ferriso, The Marilyn H. and Dr. Robert B. Pamplin Jr. Director and Chief Curator, in a prepared statement.

Christopher Rothko said in a prepared statement, "Our family is thrilled to enter into this partnership with the Museum. Portland played a formative role in my father's youth, and we are eager to share these works with the public and give Rothko a more active role in the vibrant cultural life of this city."

Groundbreaking on the Rothko pavilion is scheduled for 2018, with the project scheduled for completion in late 2020 or early 2021. The pavilion, which will have 9,840 square feet of new gallery space, is being designed by Vinci Hamp Architects, a Chicago firm that has designed several special exhibits for the museum, such as last year's "Italian Style: Fashion Since 1945" and 2012's "The Body Beautiful in Ancient Greece."

In addition to the space for Rothko paintings, the pavilion will have space for contemporary and media art, a third-floor sculpture garden, a new education and design lab, and library space.

The pavilion will also become the museum's new entrance, with the goal of guiding more of the 350,000-plus annual visitors to the museum into its Mark Building, a former Masonic temple next to the main building.

"Our studies of visitor movement have shown that fully half of our visitors don't actually make it over to the modern/contemporary wing" in the Mark Building, said Ian Gillingham, a museum spokesman, "and you have to figure that in part they didn't realize they had to go through an underground gallery to get there. That's one thing that's really going to open up the museum."

The museum's gift shop and cafe will be remodeled, Gillingham said. They will remain open during construction, though they may be temporarily moved.

The museum's last major expansion project was the Mark Building, which was purchased in the early 1990s for $2.5 million, underwent a $42 million renovation and opened in 2005.

"You're going to see the museum poised to take the next step in terms of not simply its ambition but the quality of offerings," Row said. "Particularly since I think Brian (Ferriso) has spent the last 10 years trying to buttress the foundations of the museum in terms of its educational programs and paying off some debt that it's had from previous tenures." Ferriso was named museum director in 2006, following the departure of former director John Buchanan in 2005.

Describing Rothko as a "critically important figure in art history in the 20th century," Row said the partnership with the Rothko family "promises something very, very positive for the community."

"For the museum, and therefore by extension the community, there's an elevation of value," Row said. "That collaboration with the Rothkos means you get more collaborations with other foundations and organizations that have similar value. It just widens a sphere of influence for the museum, and there's just a lot more possibility."